The invention relates generally to a device for cooling and more particularly to a device for cooling a SQUID measuring instrument that measures biomagnetic or other weak magnetic fields.
Such cooling devices include a cryostat containing a vacuum in its interior space. At least one superconducting gradiometer, as well as at least one associated SQUID, are located in the interior space and are thermally coupled via heat-conducting connections to a cryogen (i.e. a cooling medium) that is supplied from outside the cryostat. This type of cooling device is disclosed in U.S. Pat. No. 4,827,217, for example.
Superconducting quantum interferometers, so called "SQUIDs", measure very weak magnetic fields (see, for example, J. Phys.E.: Sci. Instrum., vol. 13, 1980, pp 801 to 813 or IEEE Trans. Electron Dev.. vol. ED-27, no. 10, October 1980, pp 1896 to 1908). In the field of diagnostic medicine, magnetocardiography and magneto-encephalography are examples of areas in which these interferometers can be used. In this regard, the magnetic fields produced by heart or brain currents have field strengths on the order of only about 50 pT or 0.1 pT (see, for example, "Biomagnetism--Proceedings Third International Workshop on Biomagnetism, Berlin 1980", Berlin/New York 1981, pp 3 to 31). In addition, these very weak fields, or their field gradients, must be detectable even in the presence of relatively large interference fields. To measure biomagnetic fields of such an order of magnitude, measuring instruments are known that have a single-channel or multi-channel design (see, for example, EP-B-0 111 827). These measuring instruments contain a number of SQUIDs and magnetometers or gradiometers for receiving the field signals to be detected which correspond to the number of channels. Unlike the gradiometers, the magnetometers have only a single detecting loop and can therefore be regarded as gradiometers of zeroth order. Therefore, the term "gradiometer" as used below will generally include magnetometers as well as gradiometers.
Known SQUID measuring instruments, which are also called SQUID magnetometers, have cryostats accommodating the superconducting gradiometers and the SQUIDS. However these cryostats are generally arranged above the field source of a patient to be examined. If the filling capacity of a cryostat is assumed to be about 15 to 30 liters of a cryogenic fluid such as liquid helium (LHe), which is typical for multi-channel measuring instruments with sufficiently large refill intervals, for example, then this cryostat configuration can endanger the patient who is positioned directly below it. If the vacuum insulation were to suddenly collapse or if the cryostat were suddenly damaged and the safety valves blew, a dangerous amount of cold gas would flow onto the patient. Thus, in such an emergency, the 15 to 30 liters of liquid helium would produce about 10 to 30 m.sup.3 of cold helium gas. This cold gas cannot realistically be kept away from the patient, particularly when the examination is taking place in a screening booth. Moreover, the patient may consider the positioning of relatively large cryostats over his head unpleasant.
In the cooling device for a SQUID measuring instrument disclosed in U.S. Pat. No. 4,827,217, a considerable quantity of a cryogen in a cryostat is positioned above the patient. The cryogen is located in a separate supply tank inside the cryostat. A SQUID and the associated superconducting coils of the gradiometer are configured in a vacuum space within the cryostat located below the supply tank and are thermally coupled, via parts made of good heat-conducting material, to the supply tank and consequently to the cryogen contained therein. However, the cryostat in this known cooling device, which indirectly cools the superconducting parts of the SQUID measuring instrument, is very costly to construct because of its large dimensions due to the amount of cryogen required.
WO-A-85/04489 also discloses a device for indirectly cooling a SQUID measuring instrument. In this known instrument, it is possible to position the cryostats on one side of the field source to be detected.
A cooling device for a SQUID measuring instrument, whose cryostat can be positioned on one side or below a patient, can be inferred from the publication CRYOSOUID--Proc. of 5th Cryocooler Conference (Cryocoolers 5), Monterey, U.S., 1988, pp 35 to 46. This cooling device cools the superconducting parts by a combination of an LHe bath and numerous He gas currents. The SQUIDs are cooled by an insert element filled with LHe, while the gradiometers are cooled by exhaust gas. The refill interval is about one day. The abovementioned safety problems are negligible in this known cooling device since the position of the cryostat is predetermined. Furthermore, only a relatively small LHe supply is required. However, the refill interval is relatively short. To refill the cryostat, an LHe transport container is brought at least once a day into the screening booth in which the SQUID measuring instrument is situated. In addition, subdividing the gas current into several shunted circuits causes problems for the various gradiometers. Moreover, there is no temperature regulation and it is hardly possible to provide one. Also, it is not possible to adjust the LHe vaporization rate without the application of additional measures. As with a LHe cooling bath, a cold, helium, vacuum-tight cryostat receptacle is needed.
Up until now, the superconducting parts of known SQUID measuring instruments, particularly the SQUIDs, were not cooled with refrigeration machines. Because of their electromagnetic radiation, moving magnetic parts, and vibrations, these machines can generate magnetic interference, which are directly or indirectly detected by the SQUID measuring instrument. For these reasons, refrigeration machines were absolutely avoided. Small refrigeration machines having an adequate refrigeration capability and an operating temperature of under 6K are generally known. They operate according to the so-called Linde-Claude principle (see, for example "Linde Reports from Technology and Science", No. 64: The Linde Refrigerator for Nuclear Spintomography, 1990, pp 38 to 45). Additionally, a combination of a two-stage Gifford-McMahon (GM) refrigerator for the temperature range above 10K with a Joule Thomson (JT) cycle can also be used (see the above-mentioned passage from Cryocoolers 5).
Refrigerators that can cool to a temperature of about 4K using the GM principle are also known. To accomplish this, special magnetic materials, such as Gd.sub.x Er.sub.1-x Rh are used as regenerators. Due to a magnetic phase transition that occurs even below 10K, these special magnetic materials have a considerable specific heat (see, for example, Adv. Cryog. Engng., vol. 35 b, 1990, pp 1251 to 1260). Besides the direct coupling of a refrigeration machine and a SQUID measuring instrument, (as pointed out in the above-mentioned text passage from Cryocoolers 5) other methods of thermally coupling devices requiring very low temperatures to refrigeration machines are known from the field of superconducting magnets (see, for example, Cryogenics, vol. 24, 1984, pp 175 to 178).
The problem with the prior art is that there is no cooling device with the features mentioned above that can be configured so that the cryostat does not adversely affect the field source to be detected. More specifically, the cryostat should not endanger a patient. Therefore, there should only be a relatively small amount of cryogen in the vicinity of the field source, allowing the cryostat to have appropriately small dimensions.